Electrolytic dehydrogenation of hydrocarbons



May 23, 1967 G. s. MILL 3,321,386

ELECTROLYTIC DEHYDROGENATION OF HYDROCARBONS Filed July 30, 1964 MFU-LSED 1' METAL, IODIDE i-NVENTORI GEORGE STEWART MILL M I m ms ATTORNEY United States Patent 3,321,386 ELECTROLYTI DEHYDRGGENATION 0F HYDRQfiARBONS George Stewart Mill, Pasadena, Tex., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed July 30, 1964, Ser. No. 386,309 6 Claims. (Cl. 264-62) This invention relates to an improved process for the conversion of hydrocarbons to other hydrocarbons having a higher carbon-to-hydrogen ratio.

British Patent 895,500, published May 2, 1962, discloses the use of certain metal oxides as HI acceptors in the reaction of iodine with organic compounds. Maxwell Nager, in US. 3,080,435, dated Mar. 5, 1963, proposes to dehydrogenate organic compounds by a process involving: (1) iodinative dehydrogenation of an organic compound by reaction with elemental iodine in a molten metal iodide environment to produce a dehydrogenated compound and hydrogen iodide, (2) immediately producing a metal iodide by reaction of the produced hydrogen iodide with the correspnding metal oxide or hydroxide in the dehydrogenation zone, and (3) regenerating elemental iodine from the metal iodide, either in the dehydrogenation zone or in a separate oxidation zone, by reaction with oxygen under conditions under which the metal iodide is in a molten state. Such iodinative dehydrogenative processes often have the disadvantage of producing undesirale oxidized organic compounds which decreases the amount of the desired less-saturated hydrocarbons produced.

It is the pnmaryobject of this invention to provide an improved process for dehydrogenation with iodine wherein the use of a quench following reaction is minimized or avoided or wherein the amount of iodine species in the reactor effluent is minimized or avoided. It is a further object of the invention to provide such a process promoting a desirable shift in the equilibrium of the reaction mixture. These objects will be better understood and others will become apparent from the description of the invention, which will be made with reference to the accompanying drawing wherein:

FIGURE I is a cross-sectional diagrammatic view of one apparatus for carrying out the process of the invention; and

FIGURE 11 is a crosssection of another apparatus for carrying out the process of the invention.

Now, in accordance with the present invention, an improved process has been provided for the dehydrogenation of hydrocarbons by intimately contacting the compound with a fused electrolyte of certain metal iodides and electrolytically produced iodine at a temperature in excess of about 300 C. while electrolyzing the metal iodide, whereby a second hydrocarbon having a higher carbon-to-hydrogen ratio is formed, and withdrawing the second hydrocarbon.

While the present invention does not depend on any postulated theory, it is believed that:

When a current is passed through the fused metal iodide, the latter is electrolyzed to yield elemental iodine at the anode and corresponding metal at the cathode. Upon introduction of a hydrocarbon into the fused metal iodide during electrolysis at a dehydrogenation temperature, elemental iodine as formed immediately reacts with and dehydrogenates the hydrocarbon to form a second hydrocarbon and hydrogen iodide. These components are readily conducted through the cathode zone where free metal is being formed from metal ions at the cathode. The free metal, preferably one of the more reactive metals of those listed hereinafter, reacts with the iodine species, including hydrogen iodide, excess iodine, etc. to reform the metal iodide while the second hydrocarbon and hydrogen are withdrawn through a suitable exit of the electrolytic cell.

The present invention, eliminating the use of a metal oxide as a hydrogen iodide acceptor, has all of the advantages of such an acceptor plus a number of other advantages not possessed by metal oxide. For example, the amount of iodine released is more directly controlled by the proportion of current being passed through the cell relative to the amount of hydrocarbon being introduced. This, in turn, leads to more selective and more easily controlled dehydrogenation. Since no oxygen is present in the system (other than incidental or impurities) there is no possibility for forming undesirable oxygenated prod ucts. Furthermore, if small amounts of oxide impurities are present initially, they will react with HI forming H O which will be removed from the system with the effluent. Because of this there is no possibility of depleting the activity of the system due to the formation of iodates, periodates or other oxy-iodide species. The by-product of the reaction is hydrogen, a product having more value than by-product water which is formed in the process involving metal oxides. Initially excess free metal may be introduced to insure complete trapping of iodine species. By proper cell design, the hydrogen iodide acceptor, molten metal, will always be in an advantageous place in the reactor. This is not necessarily the case in the process involving the use of metal oxide unless rapid circulation of the melt can be carried out. Finally, the metal, by virtue of its liquid state and its presence at the right position, is potentially a better hydrogen iodide acceptor than the metal oxide.

In its simplest form, the molten electrolyte comprises a metal iodide which is readily electrolyzed to produce iodine and reactive elemental metal which will readily recombine to form the metallic iodide, and the metal readily reacts with iodine species produced in the dehydrogenation process. The most useful metals for this purpose are those which not only meet the above criteria but also which form a molten metal at the temperatures encountered in the electrolytic cell, such temperatures being those required for efficient dehydrogenation to occur. In general, the preferred metals for this purpose are the alkali metals, especially lithium, sodium and potassium; mixtures of diiierent metal iodides are also useful. Mixtures are especially useful as they possess relatively lower melting points than the individual substances. It is possible, of course, to utilize a mixture of metallic iodides in which one metallic iodide is of a type yielding a molten metal at the dehydrogenation temperatures occurring within the electrolytic cell while a second metallic iodide forms only a solid metal at the cathode which is dispersed throughout the molten metal being formed there as well, Also, in addition to the metal iodide, there may be present corresponding metal halide other than the iodide; the other halide ions require a higher electrode potential than iodide for oxidation to free halogen so they remain as halide While iodide is electrolyzed to elemental iodine, e.g., to include metal fluoride which is not as easily oxidized as iodine.

The use of a molten metal iodide as the electrolyte in which dehydrogenation is conducted has a number of inherent advantages not possessed by systems involving the use of solid metallic oxides or iodides. Thus, attrition of solids is no longer a problem. Liquid-gas contacting is much more efficient than solid-gas contacting. Melting points and vapor pressures of iodides are no longer limiting. In a molten salt system, substantially all of the iodide is available for reaction. Finally, if a ceramic lining of the electrolytic cell should crack, molten salt would leak in, solidify and seal the crack thus preventing corrosive vapors from attacking any metal casing.

The invention will be better understood from a more detailed description made with reference to the figures of the drawing. FIG. I represents a simple arrangement for practice of the invention especially where molten metal formed in the electrolytic cell has a lower specific gravity than the fused metal iodide. A hydrocarbon feed from a source 1 is passed by feed line 2 to inlet 3 of electrolytic cell 4. The cell may be jacketed, e.g., by insulation or heating element 5 while the cell is filled with mol-ten metal iodide. Current is passed between anode 6 in a lower section of the cell and cathode 7 spaced above the anode while introducing the feed hydrocarbon. Under the influence of the current, elemental iodine is formed in the anode zone while elemental metal is formed in the cathode zone. This metal forms a liquid layer 8 at the top of the fused metal iodide electrolytic bath if the metal iodide is chosen so that the metal released at the cathode has a lower specific gravity than that of the fused metal iodide. This molten metal traps substantially all of the iodine species including any excess iodine and hydrogen iodide passing therethrough, the metal iodide being thus reformed and commingling with the main body of the fused metal iodide. The hydrocarbon reaction products proceed through exit conduit 9 and through products line 10 to product storage 11. Of course, provision is made in such a cell for preventing the fused metal iodide from being released downward through the stack entrance 3; this can be done by proper arrangement of lines and valves as will be readily apparent to those skilled in the related art. Also, design and operation will be selected to prevent the floating layer of metal from short-circuiting the electrodes. Furthermore, bafliing means can be inserted within the reaction zone to prevent iodine from being carried into contact with the cathode prior to reaction with the hydrocarbon.

While the invention described with reference to FIG. I eliminates any necessity for solid-gas contacting and is an improvement over the proposed use of fused metallic iodides in non-electrolytic systems under oxidative conditions, nonetheless residual amounts of iodine and iodine species may escape from the cell. In the case of low melting metals, metal vapors may also tend to escape. However, those skilled in the art are familiar with various means for trapping these vapors, e.g., a lower temperature zone wherein the metal could be vibrated back into the melt. Since the efiiciency of the process and the stringent economic requirements thereof call for essentially complete recovery of iodine species from the system, it is usually desirable to supplement the electrolytic cell by what may be termed an iodine species trap. This may comprise any one of a number of iodine species acceptors which may be either solid or liquid but in the present instance it is convenient to utilize part of the molten metal being formed in the electrolytic cell wherein the remaining products are removed by means of exit line which passes to the iodine species trap. This trap may be supplied with a portion of the molten metal being formed at the cathode.

According to FIG. II, a hydrocarbon feed is passed by feed line 21 to the electrolytic cell 22. The cell may be jacketed, e.g., by insulation or heating element 23, while the cell is filled with molten metal iodide. Current is passed between the anode 24, along one side of the electrolytic cell, and cathode 25, along the opposite side of the cell, while introducing feed hydrocarbon. The current causes elemental iodine to be formed in the anode zone while elemental metal forms in the cathode zone and rises to the top of the molten metal iodide in a liquid layer 26. A partition 27 of either impervious or semiimpervious material separates the anodic and cathodic zones, thereby preventing short-circuiting of the cell. The hydrocarbon introduced via line 21 passes through the anode zone where it contacts and reacts with the elemental iodine. The unsaturated hydrocarbon, HI and unreacted gases pass to the top of the anode zone and via a one way valve 28 into the molten metal zone wherein HI and any unreacted iodine react with the metal, thereby retaining all the iodine within the cell. The effiuent and H pass out of the electrolytic cell through line 29.

Details as to relative sizes, shapes and placement of the pieces of equipment and position for gas compressors, valves, bai'fies, fluid-cells, insulators, condensers, heaters and the like are omitted for clarity since they will be readily supplied by those skilled in the relevant art.

This invention is applicable to iodinative dehydrogenation of various compounds and is especially suitable for those hydrocarbons treated in US. 3,080,435, supra.

The feed charged to the reaction mixture may be a pure iodine-reactive hydrocarbon or it may be an iodine-reactive hydrocarbon in admixture with other iodine-reactive hydrocarbons or it may be an iodine-reactive hydrocarbon in admixture with inert diluent. An inert diluent, for example, is helium or nitrogen, which is not converted in any manner under the conditions of this invention.

The proportion of iodine being formed in the electrolytic cell should be at least about 0.05 mol per mol of hydrocarbon being introduced. Still more preferably, the amount of iodine is at least 0.2 mol per mol of hydrocarbon and it is preferred that the molten metal iodide electrolytic cell be arranged and the body of the bath be such that optimum contacting of the feed hydrocarbon with fused metal be possible.

The residence time of the hydrocarbon Within the electrolytic reaction cell will vary from one feed to another and is to be coordinated with the specific metallic iodides being utilized as well as with the temperature at which the electrolytic cell is being held. When the reaction temperature is between about 300 and 650 C., the residence time will be normally within the period from about 0.05 second to about 1 minute and preferably from about 0.1 second to about 10 seconds.

The voltage required to decompose the metal iodide will be dependent on a number of factors among which is the decomposition potential, i.e., the minimum electromotive force required to cause a steady electrolysis of the specific metal in the fused bath. This cross-cell voltage is a measure of resistance to conductivity of the fused metal electrolyte. This voltage will vary considerably depending on the metal iodide involved, but from one and one-half to four volts should usually be sufficient. Additional voltage corresponding to the electrode potential of the electrodes will depend on the specific reactivity energy of the electrodes and usually is in the range of 1.5 to 1.7 when common inert electrodes are utilized. In addition, the cell voltage and electrolyte composition can be adjusted so that the heat supplied to melt the metal salt may be obtained from the resistance of the metal salt (cross-cell voltage) and any additional voltage necessary to maintain a molten condition within the cell.

Suitable electrodes are inert to the reaction in the electrolytic cell. Such materials as graphite, carbon, platinum, palladium, silver, chromium and mixtures and alloys thereof with or without other metals, exemplify those elements which are advantageous to the instant invention. The electrodes are preferably of a perforated or grid type although solid electrodes may be utilized in conjunction with or in addition to the perforated or grid types. It is understood that DC. current is employed so that the zone of the released elements will be the same at all times.

The amount of iodine released in a given period of time depends on the current density and the anode surface. About 53.6 ampere x hours (193,000 coulombs) of electricity is required to release one mole of iodine. The current density will be adjusted for optimum results depending on the size of the electrode surface, spacing of the electrodes and the rate at which iodine is to be released.

In the operation of the novel process of this invention, the apparatus depicted in FIG. I was loaded with the lithium iodide and the temperature of the cell raised to 465 C. to melt the salt. A current of l2 arnperes from a 12 volt source was passed between the carbon electrodes to the molten lithium iodide. Propane was introduced at a rate of 60 cubic centimeters/minute. The product stream was removed and propylene was recovered therefrom in good yield corresponding substantially to the electrochemical equivalent of the electrical input.

I claim as my invention:

1. A process for conversion of a first hydrocarbon to a second hydrocarbon having a higher carbon-to-hydrogen ratio which comprises passing a current between spaced electrodes immersed in a fused salt electrolyte comprising a fused metallic iodide thereby forming elemental iodine at one electrode while introducing the first hydrocarbon into the electrolyte adjacent the electrode where elemental iodine is formed while maintaining the electrolyte at a temperature in excess of 300 C., and withdrawing the resulting second hydrocarbon.

2. A process in accordance with claim 1 wherein a voltage in excess of about 5 volts is maintained across the electrodes.

3. A process in accordance with claim 2 wherein the metal iodide comprises an alkali metal iodide.

4. In the process for conversion of a first hydrocarbon into a second hydrocarbon having a higher carbon-tohydrogen ratio, wherein the first hydrocarbon is dehydrogenated by reaction with iodine at a temperature in excess of about 300 C., the improvement comprising introducing the first hydrocarbon into an electrolytic cell, said cell comprising a molten metallic iodide with an anode and a cathode disposed therein, whereby elemental iodine is liberated at the anode and reacts with the first hydrocarbon to form the second hydrocarbon and hydrogen iodide, wherein an equivalent amount of metal is liberated at the cathode and this is contacted with the hydrogen iodide to form metal iodide, and hydrogen and thereafter removing hydrogen and the second hydrocarbon from the cell.

5. A process in accordance with claim 4 wherein the metal iodide comprises essentially an alkali metal iodide.

6. A process in accordance with claim 4 wherein the metal iodide is lithium iodide.

References Cited by the Examiner UNITED STATES PATENTS 3,080,435 5/1963 Nager 260673.5 3,114,684 12/1963 Olstowski et al. 20460 X JOHN H. MACK, Primary Examiner.

D. R. VALENTINE, Assistant Examiner. 

1. A PROCESS FOR CONVERSION OF A FIRST HYDROCARBON TO A SECOND HYDROCARBON HAVING A HIGHER CARBON-TO-HYDROGEN RATIO WHICH COMPRISES PASSING A CURRENT BETWEEN SPACED ELECTRODES IMMERSED IN A FUSED SALT ELECTROLYTE COMPRISING A FUSED METALLIC IODIDE THEREBY FORMING ELEMENTAL IODINE AT ONE ELECTRODE WHILE INTRODUCING THE FIRST HYDROCARBON INTO THE ELECTROLYTE ADJACENT THE ELECTRODE WHERE ELEMENTL IODINE IS FORMED WHILE MAINTAINING THE ELECTROLYTE AT A TEMPERATURE IN EXCESS OF 300%C., AND WITHDRAWING THE RESULTING SECOND HYDROCARBON. 